There exists a rotary damper known as a so-called unidirectional rotary damper, which generates strong damping torque for rotation in the normal rotation direction while it generates weak damping torque for rotation in the reverse rotation direction. For example, the below-mentioned Patent Document 1 discloses a unidirectional rotary damper, which is simple in structure and can be produced at low cost.
The rotary damper described in the Patent Document 1 comprises: a case having a cylindrical chamber; a rotor (i.e. rotating body) having a rotor body of a cylindrical shape and vanes, the rotor being housed in the cylindrical chamber such that its rotation axis coincides with the center line of the cylindrical chamber, to allow rotation of the rotor; viscous fluid filling the cylindrical chamber; and a lid for enclosing the rotor and the viscous fluid in the cylindrical chamber. In the inner wall surface of the cylindrical chamber, there are formed bulkheads, which are projecting toward the center line so as to form a narrow gap between each bulkhead and the outer periphery of the rotor body. Each vane protruding from the outer periphery of the rotor body toward the inner periphery side of the cylindrical chamber is formed, so as to form a narrow gap between the vane and the inner periphery of the cylindrical chamber. In each vane, there is formed a flow path from one side surface (referred to as first side surface) perpendicular to the direction of rotation of the rotor to the other side surface (referred to as second side surface). Further, an apical surface (a surface opposite to the inner wall surface of the cylindrical chamber) of each vane is attached with a seal member to close the gap between the apical surface and the inner wall surface of the cylindrical chamber. Each seal member has a check valve of an elastic body, which opens and closes the flow path from the side of the second side surface with respect to the direction of rotation of the vane.
The rotary damper described in the Patent Document 1, in the above configuration, blocks the flow paths by check valves pressed against the second side surfaces of the vanes because of the viscous fluid in the cylindrical chamber, when force for rotating in the direction from the first side surfaces of the vanes toward the second side surfaces of the vanes (i.e. in the normal rotation direction) is applied to the rotor. As a result, the movement of the viscous fluid is limited to movement through the narrow gap between each bulkhead of the cylindrical chamber and the outer periphery of the rotor body. Thus the pressure on the viscous fluid at the sides of the second side surfaces of the vanes increases, whereby strong damping torque is generated. On the other hand, when force for rotating in the direction from the second side surfaces of the vanes toward the first side surfaces of the vanes (i.e. in the reverse rotation direction) is applied to the rotor, the viscous fluid on the sides of the first side surfaces flows into the flow paths and pushes up the check valves to open the flow paths. As a result, the movement of the viscous fluid occurs also in the flow paths. Thus, the pressure on the viscous fluid on the sides of the first side surfaces of the vanes is not increased, and thus weak damping torque is generated.